The Plain Old Telephone System (“POTS”) consists of a network of local concentrations of telephone equipment, known as Central Offices (“CO”), which connect to surrounding subscribers. A subscriber's telephone is usually connected to the CO by way of two copper wires, which are referred to as the tip lead and the ring lead, in a familiar configuration commonly known as a twisted pair. FIG. 1 shows a high-level block diagram of a CO 10 and a subscriber 20.
The two-wire interface at the CO is a high voltage interface and requires high voltage circuits to drive it. Circuitry in a modem day CO 10, such as shown in FIG. 1, typically includes a software driven micro-controller 12, or, alternatively, an ASIC, interfaced with a computer 11, a low-voltage mixed-signal Integrated Circuit (“IC”) 13, and a high-voltage Subscriber Line Interface Circuit (“SLIC”) 14 connected to a twisted pair 15. The SLIC 14 is provided to feed power to a telephone 21 at the subscriber 20, transmit voice band frequencies, and detect subscriber signaling.
The connection between the CO and the subscriber is analog, and the signaling mechanism from the subscriber to the CO is the change in resistance of the telephone as it transitions between the on-hook and off-hook states. There is a large difference between the telephone's on-hook DC-resistance and its off-hook DC-resistance and, depending on whether the CO uses a voltage-feed SLIC or a current-feed SLIC, some voltage or current limiting may be required in order to protect equipment or maintain safe operation.
A DC-feed control loop is typically employed in SLICs. The purpose of the DC-feed control loop is to regulate the tip-to-ring current and voltage for all possible line conditions. In early prior art implementations, the DC-feed control circuitry was included on the high-voltage SLIC. The circuitry was entirely analog, there was no software programmability, and any change to the operating parameters usually required changing resistors and/or capacitors. Furthermore, the inclusion of control circuitry on the high-voltage part was expensive.
Consequently, newer designs of DC-feed control have transferred control circuitry from the high-voltage part to the low-voltage part and added software programmability. However, control loops remain analog and programmability is accomplished using digital-to-analog converters (“DACs”) to set analog parameters. Thus, each programmable feature requires an additional DAC, adding integrated circuit chip area and power consumption with each such additional DAC. Furthermore, analog implementations usually require more than one amplifier, and they therefore consume a large amount of power and area, and generate considerable noise. Low Idle Channel Noise is a stringent requirement imposed on DC-feed circuits, and the noise contributed by analog implementations of DC-feed control is usually dominated by low-frequency 1/f noise, again, contributed primarily by the amplifiers.